The present invention relates to a vinyl chloride resin composition having an excellent processability and being capable of giving a molded articles excellent in physical properties, and more particularly to a vinyl chloride resin composition containing as a processing aid a polymer composed mainly of methyl methacrylate and other alkyl methacrylate or an alkyl acrylate.
Polyvinyl chloride is widely used in various fields because it provides molded articles excellent in physical and chemical properties. However, polyvinyl chloride has various problems in processing, for example, the temperature range adoptable to the molding is relatively narrow because the processing temperature is close to the thermal decomposition temperature and, moreover, it takes long time to become in a molten state.
At present, many techniques to overcome the above-mentioned problems in processing are known. Representative techniques thereof are, for example, addition of a plasticizer to polyvinyl chloride homopolymer, use of a vinyl chloride resin wherein vinyl chloride is copolymerized with other monomers, and mixing of polyvinyl chloride homopolymer with other resin components.
However, by any of these techniques the processability could not be improved sufficiently with keeping excellent physical and chemical properties inherent to polyvinyl chloride. For example, when a plasticizer is added to polyvinyl chloride or when a vinyl chloride resin wherein vinyl chloride is copolymerized with other monomers is used, physical properties of molded articles prepared therefrom are remarkably altered. Main purpose of mixing polyvinyl chloride with other resin components is to reduce the melt viscosity in molding processing to thereby lower the processing temperature. This manner can improve the fluidity of polyvinyl chloride in the appearance, however, in practice the gelation of polyvinyl chloride occurs insufficiently because the kneading energy is consumed by the fluidizing. Thus, the product is inferior in physical properties to a sufficiently gelled polyvinyl chloride even though its appearance is transparent.
In order to solve the problems as mentioned above, JP-B-40-5311 proposes to incorporate as a processing aid a copolymer comprising methyl methacrylate as a main component, which has a relatively high molecular weight, into polyvinyl chloride. However, the added processing aid is apt to remain as an ungelled substance (generally called "fish eyes") in the product and, therefore, the appearance of the product is easy to be impaired. Furthermore, there is a problem that further effects desired to be produced by adding a processing aid such as enhancement of molded article gloss, improvement of secondary processability and decrease in specific gravity of molded foam, are not sufficiently exerted.
JP-B-52-49020 and JP-B-53-2898 propose to incorporate as a processing aid a two-stage polymer which is prepared by polymerizing a major amount of an acrylic acid ester or a methacrylic acid ester other than methyl methacrylate in the presence of a latex of polymethyl methacrylate or a copolymer containing a major amount of methyl methacrylate. This proposal has a certain effect on preventing generation of an ungelled substance. However, the proposed processing aid is not still satisfactory, because when the processing aid is added to a vinyl chloride resin, there often occur phenomena which are assumed to result from insufficient dispersion of the processing aid into the vinyl chloride resin, e.g., deterioration of transparency of a molded article, decrease in degree of gelation, deterioration of secondary processability such as elongation at high temperatures and the like.
In the case of using a processing aid having a very high molecular weight, it is easily expected that the effect of giving a viscosity and an elasticity to a vinyl chloride resin becomes higher. However, it is also well known that, in the case of using a processing aid the molecular weight of which is simply increased, the above-mentioned phenomena due to poor dispersibility occur remarkably.
In order to eliminate the problems, such as the generation of an ungelled substance and the deterioration of transparency, which are encountered in such a high molecular weight region, Japanese Patent No. 2,515,014 proposes to adjust the particle size of a latex of a two-stage polymer to at most 1,000 .ANG., wherein the latex is prepared by polymerizing a component containing a major amount of methyl methacrylate and subsequently adding and polymerizing a component containing a major amount of an alkyl acrylate. However, this technique cannot produce a sufficient effect for improving the gelation property and, therefore, the use of a resin composition incorporated with such a processing aid is limited in some cases.
French Patent No. 2,180,595 discloses a technique wherein a component having a relatively low glass transition temperature which mainly comprises an acrylic acid ester is used as a core for the two-stage polymerization. The object of this technique is to prevent a vinyl chloride resin from adhering to a metal surface at processing, to suppress an increase in viscosity of a vinyl chloride resin and to suppress revelation of an elasticity of a melted resin represented by Barus effect, by the use of a processing aid having a relatively low molecular weight. This processing aid has no improvement in processability and the like as achieved by the present invention.
As described above, generation of an ungelled substance and deterioration of a transparency resulting from a processing aid incorporated into a vinyl chloride resin are undesirable, because of not only exhibiting no effect of the processing aid, but also impairing excellent physical and chemical properties that the vinyl chloride resin originally possesses. If the problem of the generation of an ungelled substance is solved, it is possible to give a higher effect of a processing aid by using the processing aid in the same amount as in a conventional one and to decrease the amount of the processing aid for giving the same degree of effect as that achieved by a conventional one. Additionally, it is possible to prevent deterioration of physical properties which results from increasing the molecular weight of a processing aid and, therefore, it is possible to expect a processing aid which can be used in an improved high efficiency. In the preparation of a processing aid, dusting easily occurs when a polymer is separated from a latex. Dusting is undesirable because occurrence of dusting makes it inconvenient to handle the polymer.
It is an object of the present invention to provide a vinyl chloride resin composition having an excellent processability and being capable of giving a molded article excellent in physical properties.
A further object of the invention is to provide a processing aid which can improve the gelation property of a vinyl chloride resin by the use in a smaller amount and, resultingly, which suppresses remarkably the generation of an ungelled substance and which remarkably improves the processability without deteriorating the transparency.
A further object of the present invention is to provide a processing aid which is hard to cause dusting and, therefore, is easy to handle.
These and other objects of the present invention will become apparent from the description hereinafter.